Alternate fuels have been developed to mitigate the rising prices of conventional fuels and for reducing exhaust emissions. For example, alcohol and alcohol-containing fuel blends have been recognized as attractive alternative fuels, in particular for automotive applications. Various engine systems may be used with alcohol fuels, utilizing various engine technologies and injection technologies. Further, various approaches may be used to control such alcohol-fuelled engines to take advantage of the charge-cooling effect of the high octane alcohol fuel, in particular to address engine knocking. For example, engine control methods may include adjustment of boost or spark timing in dependence upon the alcohol fuel, and various other engine operating conditions.
Engines may be configured to direct inject a knock control fluid, such as water, into an engine cylinder. The cooling and dilution effects of the injected water may be used to address cylinder knock, as well as reduce engine NOx emissions.
One example of such a system is shown by Hellen et al. in U.S. Pat. No. 6,415,745. Therein, an amount of water is direct injected into the cylinder substantially during the intake stroke. The initiation and amount of the injection is adjusted based on the required reduction of NOx and engine operating conditions.
However, the inventors herein have recognized a potential issue with such a system. As one example, unexpected torque variations may arise based on the timing and amount of the water injection. Specifically, based on the time at which the injection is initiated and ended, the cylinder temperature at the time of water injection may vary, thereby leading to variations in water vaporization effects. For example, if the injection timing is when the cylinder is warmer, the injected water may vaporize more, leading to more displacement of air-fuel charge and a larger effect on engine torque. In comparison, if the injection timing is such that the cylinder is cooler, the injected water may vaporize less, leading to less displacement of air-fuel charge and a smaller effect on engine torque. Furthermore, if the injection timing is such that little vaporization occurs before intake valve closing, there will be little displacement of trapped air-fuel mixture and little effect on engine torque. Thus, while engine NOx is sufficiently addressed, the approach of Hellen may not be able to provide the demanded torque.
In one example, the above issue may be at least partly addressed by method of operating an engine including an EGR passage coupled between an engine intake and engine exhaust. In one embodiment, the method comprises, adjusting a direct injection of a knock control fluid (such as water) to the engine. The method further comprises, during a first fluid injection timing, adjusting an engine throttle by a first amount based on the amount of fluid injection, and during a second, later fluid injection timing, adjusting an engine throttle by a second, lesser, amount based on the amount of fluid injection.
In one example, an engine may be configured with a direct injector for injecting a knock-control fluid, such as water, into an engine cylinder. In response to engine knock, water may be direct injected into the engine cylinder to address the knock. The direct water injection may be adjusted based on engine operating conditions, such as a knock intensity, engine speed-load condition, etc., and further based on a desired engine dilution and the presence of EGR transients. This may include adjusting a timing and amount of the direct water injection based on the engine operating conditions, the knock and/or the desired dilution. As such, the timing of the direct injection may be adjusted relative to an intake valve closing (IVC) event of the cylinder. For example, the injection timing may be retarded from IVC as knocking increases, and/or as a desired engine dilution increases.
Based on the timing of the direct injection, a throttle adjustment may be performed to compensate for undesired torque variations. For example, during a first water injection timing more retarded from IVC, an engine throttle may be adjusted by a first, smaller amount based on the amount of water injection. In another example, during a second water injection timing less retarded from IVC, the engine throttle may be adjusted by a second, larger amount based on the amount of water injection. In one example, the first injection timing is after IVC while the second injection timing is before IVC, or at least a part of the second injection timing is before IVC. Herein, at the first injection timing, substantially all the water direct injected may vaporize after the intake valve has closed, and is thus unable to displace the air-fuel charge trapped in the cylinder, while at the second injection timing, some or all of the water direct injected may vaporize before the intake valve has closed, taking up more space of the air-fuel charge in the cylinder. Thus, at the first injection timing, there may be fewer effects of the injected water on the amount of intake air-fuel charge. Consequently, fewer and/or smaller throttle adjustments may be required to address the smaller torque variations. In comparison, at the second injection timing, there may be larger effects of the injected water on the intake air-fuel charge. Consequently, more and/or larger throttle adjustments may be required to address the larger torque variations.
Under some conditions, the direct injected water may be performed as a multiple injection with a transition between a timing of the multiple injections adjusted based on the engine operating conditions. In one example, the timing may be adjusted based on the knock (e.g., knock timing, knock intensity, knock nature). Thus, as the knock intensity increases, such as beyond a threshold corresponding to pre-ignition, the number of water injections performed may be increased, and further a larger number of those injections may be performed before IVC to reduce the air charge temperature and expedite knock mitigation. Herein, the throttle adjustments may be based on the number and timing of the multiple injections. In an alternate example, the timing of the transition may also be adjusted based on engine operating conditions such as an engine speed-load condition, available boost, desired dilution, etc.
Throttle adjustments may also be based on the dilution effect of the injected water. As such, based on engine operating conditions, and further based on the likelihood of engine knock, a desired engine dilution may be determined. The desired dilution may be provided using dilution affecting engine parameters such as an amount of EGR, VCT timing, a valve lift, an amount of boost, etc. The injected water may also have a dilution effect, the effect based on the amount of water injected. Thus, as an amount of water injected in response to knock is increased, one or more engine operating parameters may be adjusted based on the water injection to provide the desired dilution. For example, as the amount of water injected is increased, an amount of EGR may be correspondingly decreased so that the desired dilution is provided.
While the above example illustrates throttle adjustments based on the dilution effect of water, it will be appreciated that this is not meant in a limiting sense and that in alternate examples, the injected fluid may be an alternate knock-control fluid, such as a fuel, a fuel blend, water, other inert fluids, ethanol, methanol, other alcohols, gasoline, or a combination thereof. Herein, the throttle adjustment may be based not only on the timing and amount of the direct injection of the knock-control fluid, but also based on a combination of the displacement and evaporation effects of the fluid. In one example, the combination of effects may be inferred from the molar composition of the injected fluid. For example, where the injected fluid is a blend including an alcohol fuel, the molar composition may be based on the volumetric alcohol ratio of the constituents in the blend, as well as their molecular weights and densities. Thus, as the evaporative cooling effect and/or alcohol content of the injected fluid increases, the air-fuel charge temperature may decrease and the density may increase, and thus throttle compensation may be used to decrease intake manifold pressure to achieve the desired torque. In another example, where the injected fluid is a blend including water or other inert fluid, the air-fuel mixture may be partially displaced by water injected before IVC, and thus throttle compensation may be used to increase intake manifold pressure to achieve the desired torque. In yet another example, the effects of evaporative cooling and displacement by inert vapor may partially offset each other. Additionally, an adjustment may be required in the amount of spark advance, boost, VCT, and/or EGR.
In this way, by direct injecting knock control fluid into an engine cylinder, knock may be addressed. By adjusting a throttle position based on the timing and amount and composition of the direct injection, and thus the vaporization and air-fuel charge displacement effects of the injected water, torque variations arising from the fluid injection may be better addressed, thereby improving engine performance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.